1. Field of the Invention
The present invention relates to an arrangement for aligning both light beam paths or both light beams of at least two laser diodes on a common spot.
2. Prior Art
In an especially effective process for pumping a solid state laser semiconductor laser diodes (LD) whose emission spectrum is substantially narrower than that of conventional lamps are used instead of the conventional lamps. The semiconductor laser diodes (LD) are selected so that the wavelengths of their emission spectrum coincide exactly with the pumping light wavelengths for the solid state laser to be pumped. According to the state of the art several different arrangements of the laser diodes in relation to the solid state laser crystal to be pumped are conventionally employed.
In the transverse arrangement the pumping light is radiated across or transverse to the laser beam of the solid state laser in the laser crystal. This sort of coupling has the advantage that many pumping laser diodes can be arranged over the length of the laser crystal and thus the total pumping power and thus also the laser power can be increased. The laser system is scalable in regard to its power, i.e. the laser power grows in proportion to the number of laser diodes arranged along the length of the laser crystal. However it is disadvantageous that the entire cross section of the laser crystal is irradiated by the laser light. Pumping power is lost in that portion of the laser crystal in which no laser light is produced. On the other hand, excitation of the higher transverse modes, which are not desired in those applications in which laser radiation in the transverse fundamental TEM00 mode is required, is permitted by a too large cross section of pumped laser material.
In the longitudinal pumping arrangement the pumping light is coupled in the direction of the laser beam of the solid state laser to be pumped. With this sort of excitation the pumping light may be exactly concentrated in the volume in which the laser light travels. Because of that the pumping efficiency of the coupling between the pumping light and the laser light is substantially improved in relation to the transverse coupling and on the other hand a smaller cross section can be pumped so that only the transverse fundamental mode TEM00 is excited. The possibilities for pumping light. coupling by means of a greater number of pumping light sources are however considerably limited in comparison to the transverse pumping arrangement. The pumping light can be coupled in via the front and/or rear laser mirror. At least one of the laser mirrors is made with a dichroic reflective surface, that is its surface is reflecting for laser light and transparent for pump light. In a plate-like laser crystal, in which the laser light is reflected in a crooked or folded beam path at the lateral surfaces of the crystal, also the pumping light can be coupled in from there in the direction of the laser beam, in as much as the reflections at these locations are not total reflections. The coupling locations or positions for the pumping light must have a dichroic coating.
Since the polarization direction of the light is not of significance for the pumping effect, two pumping light sources with crossed polarization directions can be combined with respective polarization radiation combining devices at the respective coupling locations. This arrangement of pumping light sources however results in considerable expense because of the required coupling optics and beam combining devices, which must be arranged around the entire periphery of the laser crystal. Proposals have however already been made in the art regarding how to avoid these disadvantages. Those proposals take advantage of the fact that slight variations in the beam propagation parameters of the pumping light and the laser light are possible, since the pumping light is strongly absorbed in the laser crystal in contrast to the laser light, so that most pumping energy is absorbed in the focused region when the pumping light is focused in the laser crystal. Because of that it is possible that several laser diodes can send a beam in via one of the above-mentioned pumping light entrances. It is presumed however that the individual laser beams have only very slight variations in their irradiation directions and in their irradiated points. Difficulties occur because the required high power pumping laser diodes have a strongly elliptical beam portions with a width of typically 100 xcexcm and a height of typically 1 xcexcm and correspondingly they propagate with elliptical radiation characteristics with a vertical spreading of typically 40xc2x0 and a horizontal spreading of 10xc2x0. For beam formation cylindrical or anamorphotic image-forming components are required because of the elliptical nature of the laser beam. According to the state of the art different proposals were made for these purposes (see e.g. U.S. Pat. No. 5,557,475) with complicated spatial arrangements of mirrors and prisms. The individual beams of the pumping laser diodes guided together can be then either coupled directly into the laser crystal or coupled by means of optical fibers with stepped profiled cores and sufficient core diameter (xe2x89xa7200xcexcm) and guided into the laser crystal. It is disadvantageous that a large number of complicated optical components must be used which must be mounted and adjusted in three dimensional space with a high accuracy and which require expensive mounting devices for a stable assembly.
It is an object of the present invention to provide an arrangement for aligning the beam paths of respective laser beams of a group of laser diodes on a common spot, especially for pumping a solid state laser.
According to the invention the arrangement for aligning beam paths of respective laser beams from at least two laser diodes on a common spot includes means for providing a bend in each beam path between the laser diode producing the laser beam traveling along that beam path and the common spot, a respective cylindrical lens provided in each beam path between the laser diode and the bend associated with that beam path, a respective focusing lens provided in each beam path between the bend and the common spot associated therewith, means for laterally moving at least one focusing lens, advantageously all of the focusing lenses, for adjustment of the beam path in which the focusing lens or lenses are arranged, so that the beam paths converge on the common spot and a common supporting substrate on which the laser diodes, the cylindrical lenses and the focusing lenses are mounted.
The arrangement according to the invention avoids the disadvantages of the prior art and provides a simple and automatic adjustment and assembly of the laser diodes, especially pumping laser diodes and their associated optical components.
Further advantageous features are set forth in the dependent claims appended hereinbelow and are described in the following detailed description.